This invention relates to a servo valve of a sliding-valve type having sliding and countersliding parts which can be moved relative to one another, having a pressure connection and at least one low-pressure connection as well as a motor connection which can be connected with the pressure connection by way of a first gap between a control edge on the sliding part and an assigned control edge on the countersliding part and can be connected with the low-pressure connection by way of another gap between another control edge on the sliding part and an assigned additional control edge on the countersliding part, one gap respectively being expanded and the other gap being contracted during control strokes or control angles of the sliding part relative to the countersliding part.
Servo valves of this type are generally known and are used, for example, in power steering systems of motor vehicles for controlling the servo motor assigned to the power steering system. In this case, the servo motor is always constructed as a double-action piston-cylinder element which is controlled by way of a servo valve with two motor connections which are arranged on mutually parallel hydraulic paths between the high-pressure and low-pressure connection of the servo valve. The control gaps assigned to these motor connections change their cross-sections during control strokes of the servo valve in such a manner that, depending on the direction of the control stroke, a more or less large pressure difference occurs between the motor connections in one or the other direction and the servo motor can correspondingly generate a more or less high power in one or the other direction.
According to German Patent Document DE 34 28 727 A1, servo valves of power steering systems can be constructed as rotary slide valves with a sliding part which can be rotated within a rotary sliding bush. These parts are arranged within the drive connection between the hand steering wheel and the steered vehicle wheels in such a manner that they rotate more or less intensively relative to one another in one or the other direction as a function of the forces transmitted between the hand steering wheel and the steered vehicle wheels so that the servo motor can generate a servo force which is a function of the amount of the transmitted forces and by which the manual force noticeable and to be applied at the hand steering wheel is reduced.
In principle, it is desired and known to change the characteristics of a power steering system as a function of the parameters to be given. For example, it can be provided to let the power steering system operate at an increased speed with a rising sluggishness, that is, with an increased manual operating force. The servo valves which have been used in this context have a comparatively high-expenditure construction.
From German Patent Document DE 39 05 636 A1, slide valve is known which is constructed as a 3/2-valve and in which a valve bore is provided in a housing and valve connections are provided which branch off the housing in the transverse direction, in which case the connection which is in the center in the longitudinal direction of the valve bore communicates with a hydraulic assembly to be controlled, and the two other connections lead to a pressure source or to a relatively pressureless reservoir. A slide, which is arranged within the valve bore and is axially displaceable, makes it possible by means of a corresponding axial displacement, to block the center connection with respect to the two other connections or to connect it in each case with one of the two other connections, in which case, depending on the position of the slide, the connection has a more or less large throttle resistance. The mouth of the center connection into the valve bore may have a noncircular shape, for example, the shape of a square, whose diagonal line is aligned in parallel to the axis of the valve bore. When the slide is slid out of its position blocking the center connection, an opening to the center connection with a triangular cross-section is first opened up whose size increases when the slide is adjusted further.
It is an object of the invention to provide a new and advantageous possibility of a parameter-dependent change of the control characteristics of a servo valve.
According to the invention, this object is achieved in that the cross-section of at least one gap which exist in the normal position of the servo valve can be changed by the displacement of its control edges relative to one another in a transverse direction with respect to the direction of the control strokes.
The invention is based on the general idea of changing the geometry of the respective gap as a result of an additional movability of the control edges transversely to the direction of the normal control strokes. This has the result that, during a control stroke, the throttle resistance of the gap changes in a changed manner and a different functional relationship is achieved between the pressure levels occurring at the engine connections and the control stroke. This means therefore that the characteristic of the progression of the pressure at the motor connection or motor connections changes during control strokes.
According to a first embodiment of the invention, the sliding part and the countersliding part respectively can be displaced relative to one another transversely to the control stroke. In the case of a servo valve constructed according to the type of a rotary slide valve, this means that the rotary sliding bush and the rotary sliding part, which is rotated relative to the rotary slide bush during control strokes, can be axially displaced with respect to one another.
In a second embodiment, a control edge part which is movable transversely to the control stroke is arranged on the sliding or countersliding part.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.